In optical communications systems, it is very critical to maintain optical signal levels at their correct power settings. In particular in wavelength division systems (WDMs), many impairments can cause some channels to have power levels that are too high or low. In long distance WDM systems in which optical signals generate nonlinear effects such as self-phase or cross-phase modulation, it is desirable to ensure that the power levels in each single span of the system remains well-equalized. The optical power levels are monitored by optical instruments such as optical spectrum monitors (or analyzers) and optical power meters.
Lock-in signal detection is commonly used to improve the sensitivity of these optical spectrum monitors and optical power meters. The method used is to chop the incoming optical signal and synchronously detect the electrical signal after the optical receiver. This enables a large reduction of electronic noise interference from the receiver, particularly in the presence of 1/F noise or drift in dc-coupled amplifiers. Conventional optical choppers comprised of either slotted rotating discs or resonant vanes are limited to chopping rates of about 10 KHz and limit the detection bandwidth to about 1000 Hz.
Many of the optical signal impairments that arise in a WDM system have fast transients, e.g., those caused by WDM add/drop or other protection switching operations. Consequently, there appears to be a strong need for an optical signal monitor having an increased detection speed and bandwidth.